Why are you constantly being taken in by saviors, leaders and friends who seem like they can do no wrong — until they let you down? Blame the Halo Effect. Turns out that once you've given someone a halo in one area, it's almost impossible to fit him or her for a pair of horns in any others.

One good trait, if sufficiently emphasized, will bleed over into everything else you do — provided you work it right.

Edward Thorndike was an educational psychologist in the 1920s. Part of his job involved watching how one set of people evaluated another set of people - generally people in teaching positions evaluating students. Over time he noticed that there was a problem. Teachers tended to favor certain students, and rate them highly in all areas — even ones in which the student was unremarkable. Everyone has favorites, but how could even obvious deficiencies be overlooked? And did the teachers really believe what they were saying?

He wrote about his findings in a paper called The Constant Error in Psychological Ratings. In it he published the results of of military officers evaluating the soldiers under their command. He found that no soldier was what people in the literary world would call a "complex and multifaceted character." People were all bad, all good, or all middling, especially if they had a few outstanding characteristics. "The Halo Effect," he said, was the tendency for one good quality to shine a pleasant light on everything else.

Several more tests have been done on the subject, and they seem to confirm that we're quick to hang a halo on anyone who has a single good quality. An experiment was done in 1970, in which students were told to watch a tape of a lecturer and evaluate it. Actually, they were watching one of two tapes of the lecturer — one in which he was warm and welcoming to students and one in which he was strict and unfriendly. The students rated the warm lecturer as more attractive, as having a better accent, and his physical gestures as more appealing.

Many studies have focused primarily on how the Halo Effect pertains to attractiveness. Another test had volunteers rating people whose photos they saw in everything from conventionality to whether they would have lifelong happiness. The more attractive people always scored higher. Studies in which people were shown pictures of people and asked to grade papers supposedly written by them also favored the pretty. Even mock-jurors were more likely to let the beautiful people go, thinking they couldn't be guilty of a crime. (More recent studies have shown, however, that attractive people of the same sex as the grader are rated more harshly than unattractive ones.)

The Halo Effect has guided everything from movie stars' careers to elections to knock-off marketing. If sticking an Apple logo on a brand didn't cause people to think more highly of it, knock-off stores wouldn't do it. (Hell, Apple wouldn't even do it.) Stars are considered intelligent if they play enough intelligent characters. And politicians strive to cultivate an image — the brilliant newcomer, the everyman, the tough military leader, the sweet grandfatherly type — in the hopes that it will emphasize their best qualities and raise them above the suspicion of their worst.

But once the halo comes off, there can be huge problems. In his 2007 book, economist Phil Rosenzweig noted that companies do well because of a number of reasons. Their financial success, however, is often attributed to their leaders, or their marketing team, or anyone who will fit the halo. That can mask problems. And once the success evaporates, the halo becomes horns, and people are willing to drive the devil out without looking at the actual problems that will continue long after one person is gone.

Even the most clear-eyed of us is guilty of emphasizing positive traits of people we're favorably disposed towards. That's often the only way to keep a friend, a co-worker, a roommate, or a spouse. But when it comes to a CEO, a pilot, or a brain surgeon, you sometimes need to whisk the halo off of someone's head. If only there were a study to show us how.